Polymer processability evaluation through on-line image processing

ABSTRACT

This invention discloses the use of one or three cameras equipped with a charge coupled device (CCD) sensor and of laser detectors for characterizing respectively in two or three dimensions, the behavior of an extrudate at the exit of a die, said CCD camera(s) being synchronized with a flash by the laser detectors.

This invention relates to the use of charge-coupled device (CCD) camerasto characterise the behaviour of a molten polymer-as it exits a die.

The degree to which a resin swells or recovers when exiting theextrusion dies is an extremely important resin characteristic in mostprocessing applications. In blow moulding, die swell is critical due tothe relationship between moulds and down-stream equipment and because ofthe relative swell exhibited by the resin during processing. Eachapplication requires a preferred amount of swelling: too little swellingmay result in lack of strength or lack of toughness, whereas too muchswelling may result in waste of material or in curtaining. It istherefore highly desirable to be able to correlate swell behaviour withresin formulation, operating conditions and die design.

In the blow moulding process, a strand of hot polymer is trapped withina mould and blown into the configuration of the mould. The shape of theparison as the mould closes determines the physical characteristics ofthe finished product. The design of the extrusion die influences theamount of polymer exiting the die and is thus a very importantparameter. In addition, as the polymer extruding from the die is aviscoelastic material, it undergoes the effects of orientation acquiredduring extrusion through the die as well as the effect of gravity. Theswell and drawdown occurring after the polymer has emerged from the diedetermine the distribution of polymer in the parison and therefore thefinal characteristics of the product and the cycle time. The swell anddrawdown have thus been studied extensively. For example Beynon andGlyde (Beynon, D. L. T. and Glyde, B. S., “The swelling and fracture ofpolyethylene melts.” In British Plastics, 414, September 1960.) havestudied the swell has a function of shear rate, temperature and flowrate. Wechsler and Baylis (Wechsler, R. L., and Baylis, T. H., “Blowmolding polyethylene”, Part I, in Modem Plastics, 107, 218, May 1959 andPart II in 115, 127, June 1959) have studied the swell as a function ofmelt temperature and extrusion rate.

Several experimental techniques have been developed to study swell andsag. As swell is a manifestation of the melt viscoelasticity, it is atime-dependent quantity. Furthermore, at the same time that it isswelling, the parison is also sagging under the influence of gravity,and its observed dimensions reflect the combined phenomena. It is thusdesirable to be able to measure swell independently of sag. Methods formeasuring swell include the pinch-off mold as described in Sheptak andBeyer (Sheptak, N. and Beyer, C. E., “Know your parison.” In SPEJournal, 190, February 1965). Other techniques are off-line methods andinclude a photographic method yielding parison diameter swell anddrawdow, a photographic technique yielding relaxation data, a multiplepinch-off device yielding material distribution and thickness. Thephotographs were then analysed and the parison had to be reconstructedfrom the photographs; this exercise proved to be difficult and notwithout errors.

Optical methods using either beam deflection or spectral attenuation aredescribed in Cielo et al. (Cielo, P., Lamontagne, M. and Vaudreuil, G.,in ISA Trans., 27, 1, 1988). They are accurate to measure the thicknessof plastic sheets but do not take into account the parison geometry.

The most precise method consists of extruding the parison into an oilbath having the same temperature and density as the melt and to takephotographs at several intervals or to monitor the process using aphotodiode array (Garcia-Rejon, A., and Dealy, J. M., in Polym. Eng.Sci., 22, 158, 1988 or Kamal, M., R., Samara, M., in Adv. Polym.Technol., 8, 367, 1988). These methods are difficult to use incommercial applications and the influence of oil on swell is not wellunderstood.

The effect on the swell of the resin's molecular weight distribution, ofthe die configuration, of the temperature and of the flow rate have beenstudied and discussed by Swan et al. (Swan, P. L., Dealy, J. M.,Garcia-rejon, A., and Derdourt, A., in Polym. Eng. and Sci, 31, 705,1991).

In a recent artist by Jivraj et al.(Jivraj, N., Sehanobish, K.,Ramanathan, R., Garcia-Rejon, A;, and Carmel, M., in The 2001 Conferenceof the PPS held in Montreal), the relationship between the resin's basicrheological properties and the parison behaviour have been studied.

The sag has been studied by the same pinch-off mould and photographicmethods as described for the swell.

In addition, the swell and sag have been studied for several die designsby the capillary rheology approach. It has been found that forcomparable shear rate at the die exit the flow kinematics of thematerial exiting the different dies are not comparable. Changing thelength (L) over diameter (D) ratio L/D of the dies did not bring abetter agreement between the divergent observations.

The swell has also been studied by the “scrap” method, whereby the scrapis not detached from the parison as seen in FIG. 1. The diameter of theparison and thus the swell is approximately twice the width of thescrap.

Another important parameter is the melt fracture that influences thetexture of the finished product. Melt fracture is a loosely defined termthat has been applied to the various forms of extrudate roughness ordistortion that are encountered at high extrusion rates for all polymermelts. It is used to describe small-scale roughness, or rippling, orsharkskin, or a very regular helical screw-thread extrudate, or any typeof irregular extrudate. Generally, these defects and irregularities areinspected by human eye resulting in imprecise evaluation and in longdelays between the detection of the imperfection and the machine shutdown.

Die deposit or die built-up consists of small drops of low molecularweight polymer ejected from the parison just after exiting the die. Itis also evaluated visually and therefore with the same bias anddrawbacks as the melt fracture estimates.

There is thus a need to characterise quickly and rigorously the moltenpolymer behaviour at the exit of a die.

The present invention provides a method for obtaining instantaneousinformation in two or three dimensions on the die combined swell and sagof the extrudate as a function of the extrusion conditions such as forexample the shear rate, the temperature or the die design and as afunction of the polymer structure or type.

The method further provides instantaneous information on the “onlyswell” and “only sag” components of the extrudate.

The method also provides on-line information on the surface texture ofthe polymer.

In addition the method allows instantaneous detection of die built up.

The method yet provides information on the rheological properties of theresin.

Accordingly, the present invention discloses the use of one or threecharge-coupled device (CCD) camera(s) and laser detectors forcharacterising instantaneously, respectively in two or three dimensions,the behaviour of a molten polymer at the exit of a die, said CCDcamera(s) being synchronised with a flash by the laser detectors.

A charge-coupled device is a light sensitive integrated circuit thatstores and displays the data for an image in such a way that eachpicture element (pixel) in the image is converted into an electricalcharge, the intensity of which is related to the intensity of lightstriking it. CCDs can be included in both still and video cameras. Inthe present invention still cameras are used for optical characterrecognition. The captors presently used are 16-bit CCD sensors workingin black and white: they provide 2¹⁶ shades of grey.

In the high performance two-phase charge-coupled sensors of the presentinvention a transparent electrodes replaces one of the polysilicongates. The transparent gate is less absorptive then polysilicon and itsindex of refraction provides a better match between the overlying oxideand the silicon substrate than that of the polysilicon, resulting inless reflective loss. The sensors are built with a true two-phase buriedchannel CCD process that is optimised for operations in multi-pinnedphase (MPP) mode for low dark current. The low dark current produces thebest signal-to-noise ratio when sensors are operated at low signallevels. The true two-phase architecture provides many advantages such asprogressive scan, square pixels, high charge capacity and simplifieddrive requirements. The photon-to-electron conversion ratios of mostCCDs are similar, but the size of the photosensitive area makes theirperformance unequal. Responsivity is a measure of the signal that eachpixel can produce and is directly proportional to the pixel area. As theresponsivity of the pixel increases, the same amount of signal can becollected in a shorter time or conversely, more signal can be collectedduring a fixed exposure time. In addition, with low-level illumination,the image has a higher signal-to-noise ratio and appears less grainy.Larger pixel areas also help improve the dynamic range because they holdmore charge and are thus not saturated quickly with bright objects.

The method of the present invention is used in the process itself andcan be used for characterising molten polymers. The extrusion parametersand the properties of all thermoplastic resins such as for examplepolyethylene (PE), polypropylene (PP), polystyrene (PS) polyvinylchloride (PVC), polyamide (PA), polymethyl methacrylate (PMMA),polyoxymethylene (POM), acrylonitrile-butadiene-styrene (ABS),polycarbonate (PC), polyacrylonitrile (PAN), styrene-acrylonitrile(SAN), ethylene vinyl acetate (EVA) can be characterised with thepresent CCD camera system. Preferably, PE and PP have been used and morepreferably PE.

The method can be used in the process control system of blow-mouldingmachines or for pipe or profile applications. It can also be used forcharacterising the resin by the extruder rheology method.

The shear rates that can be evaluated with this method range from nearzero up to 30,000 s-1.

All die types such as for example annular die or flat die can beevaluated.

The present invention also discloses a method for characterising anextrudate flowing under the die tooling that comprises the steps of:

-   -   providing one vertically moving laser or several vertically        aligned lasers that detect the lower edge of the extrudate        exiting the die and that emit successive digital signals upon        detection of said lower edge;    -   providing a micro controller that receives the successive        digital signals from the vertically moving laser or from the        several vertically aligned lasers;    -   providing a flash that is activated by the micro controller;    -   providing a CCD camera that is activated by the micro controller        and that is synchronised with the flash;    -   recording the time at each step;    -   recording the digital information at each step;    -   providing a software that calculates instantaneously the        equation for the swell in diameter and in weight and the sag of        the extrudate.

The time can be recorded with an accuracy of 1/1,000 sec.

The flash duration determines the speed at which the picture remainsclear and is thus a key factor of the system. It is preferred to workbetween 1/16 and 1/64 of the flash power. The flash duration is thuspreferably within the range of 1/9, 100 to 1/28,000 sec. It is thuspossible to obtain up to 20 images/s and therefore to work with a linearspeed of the parison or melt strand of up to 2 m/s.

The dynamic level of the camera and laser are also important parameters.The laser can be displaced at a speed of up to 2 m/s and it can beadjusted with an accuracy of the order of the mm.

The method according to the present invention may also comprise theadditional step of providing a feedback software, capable of adjustingthe parameters of die, temperature and the shear rate in order toinstantaneously optimise the extrudate's properties.

LIST OF FIGURES

FIG. 1 is a photograph of the parison with the “scrap” still appendedthereto. It is used for determining the diameter swell, defined as twicethe width of the scrap.

FIG. 2 represents a sequence of photographs of the parison at severaltimes after exiting the die. This sequence of photographs enables thesubsequent determination of the sag and swell and the determination ofthe onset of melt fracture (OMF).

FIG. 3 represents the result of a global fitting in x and y for thediameter swell wherein x represents the vertical distance from the dieexit and is expressed in cm and y represents the lateral distance fromthe vertical axis and is also expressed in cm. The equation for y as afunction of x is calculated instantaneously by a polynomial fit ofdegree 3.

The upper curve yH is represented by the third order equationyH=299.045+0.236x−7.28.10⁻⁴ x ²+5.079.10⁻⁷ x ³

The lower curve yL is represented by the third order equationyL=225.176−0.106x−2.17.10⁻⁴ x ²+5.24.10⁻⁷ x ³

FIG. 4 represents the combined swell and sag curves obtained bypolynomial fit of the parison diameter as a function parison length atseveral times after exiting the die. The parison diameter and length areexpressed in cm. The measurements were carried out at a temperature of200° C. and with the die n° 10.

FIG. 5 represents respectively the “only swell” and “only sag” curves asderived from the combined swell and sag curves of FIG. 3. The parisondiameter expressed in cm is plotted as a function of parison length alsoexpressed in cm respectively for the combined sag and swell, for theideal extrusion (a tube having the die diameter), for the “only swell”and for the “only sag”.

FIG. 6 represents the one-camera set up for the two-dimension polymercharacterisation wherein the laser activates a signal when the lower endof the extrudate exiting the die interrupts the laser beam crossingbetween said laser and a laser reflector and said signal is transmittedvia a computer synchronously to the CCD camera and to the flash.

FIG. 7 represents the three-camera set up for the three-dimensionpolymer characterisation. It operates exactly like the two-dimension setup.

FIG. 8 represents the vertically moving laser system.

FIG. 9 represents the system of several fixed vertically aligned lasers.

FIG. 10 represents a photograph of the extrudate's diameter swell at adistance x=60 pixels from the die exit.

FIG. 11 represents a photograph showing incipient melt fractures on theextrudate.

The data retrieved are the die swell both in diameter and in weight, thesag, the concentration and type of melt fracture and the concentrationof die built up: they are all recorded as a function of shear rate andtemperature for a defined die design. The shear rate can be modifiedeither by changing the die gap for a fixed output or by changing theoutput without changing the die gap.

It is also possible to derive information on the characteristics of theresin such as for example the relaxation time defined as the timenecessary to suppress the effect of orientation acquired by the moltenmaterial during its passage through the die.

The swell in diameter is defined as the ratio of the parison diameter tothe die diameter.

The swell in diameter at time t is evaluated through image processing.The software calculates instantaneously the combined swell and sag curveby fitting the observed parison wall, at time t after exiting the die,with a polynomial of the general formula:y=ax ^(n) +bx ^(n−1) +cx ^(n−2) + . . . +zwherein x is the distance from the die exit, y is the parison diameterand n is an integer of from 3 to 11, depending upon the accuracyrequired. FIG. 2 represents photographs of the parison at several timesafter exiting the die and FIG. 3 represents a polynomial fit of degree 3for one of these photographs.

The swell and sag are additive phenomena leading to the final parisondiameter the swell increases the parison diameter whereas the sag tendsto decrease it. FIG. 4 represents the combined swell and sag expressedby a polynomial fit of the walls of the parison at several times afterexiting the die.

The lower end of the parison is not affected by sagging as there is nomaterial attached below and therefore no stretching caused by gravitypull. The diameter evolution of the lower end of the parison gives thusthe true swelling factor and it leads to an extrapolated “only swellcurve”. A tube having the die dimension represents the “ideal” extrusioncurve. The “only sag curve” is calculated asSag=observed+ideal−swellas represented on FIG. 5.

The evolution of the swell phenomenon can be fitted by the exponentialequationD=Do+(D∞−Do)·(1−exp(−t/λ))wherein D is the parison diameter at time t, Do is the external diediameter, D∞ is the final parison diameter and λ is the characteristicrelaxation time related to the swelling phenomenon.

The swell in weight is defined as the weight of a parison ofpredetermined length as a function of shear rate. The swell in weight Swis the product of the swell in diameter Sd and the swell in thickness StSw=Sd×St

The parison weight or swell in weight decreases with increasing shearrate as the die gap is progressively closed in order to increase theshear rate. The swell in thickness is given by the formulaSt=m/(πpLh(Do−h))·Do/Dwherein p is the resin's density, h is the die gap, Do is the externaldie diameter, D is the final parison diameter, L is the parison lengthand m is the parison weight. It increases with increasing shear rate.

The melt fracture and die built up are determined by image processing.The sensors used in the present invention provide over 30,000 shades ofgrey allowing instantaneous detection of irregularities in theextrudate.

The present invention thus provides a novel system capable of providingin two or three dimensions instantaneous and simultaneous information onthe parameters characterising the behaviour of extrudates at the exit ofa die. The information is directly related to the process and is basedon shear rate. It can also provide information on the resin itself.

The same camera installation can also be used to study the deformationof a pinched paraison subjected to internal pressure or of a preform ininjection stretch blow moulding.

EXAMPLE

The CCD sensor used in the present application is a megapixelprogressive scan interline CCD with on-chip circuits commercialised byKodak.

It has the following parameters:

-   -   architecture: interline CCD, progressive scan, non-interlaced    -   pixel count: 1000(H)×1000(V)    -   pixel size: 7.4 microns(H)×7.4 microns(V)    -   photosensitive area: 7.4 mm(H)×7.4 mm(V)    -   output sensitivity: 12 microvolt/electron    -   saturation signal: 40,000 electrons    -   dark noise: 40 electrons rms    -   dark current (typical): <0.5 nA/cm²    -   dynamic range: 60 dB    -   quantum efficiency at 500, 540, 600 nm: 36%, 33%, 26%    -   blooming suspension: 100×    -   image lag: <10 electrons    -   smear: <0.03%    -   maximum data rate: 40 MHz/channel (2 channels)    -   integrated vertical clock drivers    -   integrated correlated double sampling (CDS)    -   integrated electronic shutter driver

The high performance 15 bit CCD sensor with transparent gate electrodeprovides 32768 unsigned levels of grey, allows the acquisition of about10,000 frames/s and covers a broad spectrum of from 400 to 1000 nm.

The installation is represented in FIG. 6 for a two-dimensionacquisition working with a single camera system and in FIG. 7 for athree-dimension acquisition working with three cameras. The successivepositions of the extrudate's lower end as a function of time can bedetermined either by a single laser moving along a vertical axissimultaneously with the extrudate as represented on FIG. 8 or by severalvertically aligned fixed lasers as represented on FIG. 9.

The extruder was the blow moulding machine, Battenfeld VK1-4.

The die swell results are represented in FIGS. 2, 3 and 10.

FIG. 11 represents the melt-fracture onset. The very high sensitivity ofthe CCD sensors allows objective and early detection of all surfaceirregularities.

The present technology provides an outstanding gain in accuracy,completeness and rapidity.

1. A method for characterizing an extrudate flowing under die toolingcomprising: (a) emitting a plurality of laser signals at a plurality ofvertically displaced locations from a laser system responsive todetection of the lower edge of an extrudate exiting from a die to emitsuccessive digital signals upon the detection of said lower edge; (b)applying said digital signals successively from said laser system to amicro controller; (c) activating a flash by said micro controller; (d)providing at least one camera with a CCD sensor that is activated by themicro controller and synchronized with the flash; (e) recording thetimes of said successive laser signals and digital signals; and (f)providing a central processor that instantaneously calculates anequation For the combined swell and sag curve of the extrudate andgenerates separate sag and swell components corresponding to said curve.2. The method of claim 1 wherein said plurality of laser signals aregenerated by a laser which is moved vertically as said signals aregenerated.
 3. The method of claim 1 wherein said plurality of lasersignals are generated by a plurality of vertically displaced lasers. 4.The method of claim 3 wherein said plurality of laser signals aresequentially generated by successively positioned vertically displacedlasers.
 5. The method according to claim 1 wherein the CCD sensor is atwo-phase charge-coupled sensor with a transparent electrode.
 6. Themethod according to claim 2 wherein said laser is moved vertically at aspeed of no more than 2 m/s.
 7. The method according to claim 1 whereinthe duration of said flash is within the range of 1/9,100 to 1/28,000second.
 8. The method according to claim 1 further comprising providingfeedback software which functions to adjust production parameters of diedesign, temperature and shear rate.
 9. The method according to claim 1wherein said extrudate is a thermoplastic polymer.
 10. The methodaccording to claim 9 wherein the extrudate is selected from the groupconsisting of polyethylene, polypropylene, polystyrene, polyvinylchloride, polyamide, polymethyl methacrylate, polyoxymethylene,acrylonitrile-butadiene-styrene, polycarbonate, polyacrylonitrile,styrene-acrylonitrile and ethylene vinyl acetate.
 11. The methodaccording to claim 1 wherein said extrudate is selected from the groupconsisting of polyethylene and polypropylene and mixtures thereof. 12.The method according to claim 1 wherein said extrudate comprisespolyethylene.
 13. The method according to claim 1 further comprisingproviding three of said cameras and characterizing the behavior of saidextrudate exiting said die in three dimensions.
 14. The method accordingto claim 1 wherein a single camera is employed to characterize thebehavior of said extrudate exiting from said die in two dimensions. 15.The method according to claim 1 wherein the onset of melt fracture ofsaid extrudate is detected.
 16. The method according to claim 1 furthercomprising calculating the relaxation time of said extrudate.